High thermal expansion, sealing glass

ABSTRACT

A glass composition for hermetically sealing to high thermal expansion materials such as aluminum alloys, stainless steels, copper, and copper/beryllium alloys, which includes between about 10 and about 25 mole percent Na 2  O, between about 10 and about 25 mole percent K 2  O, between about 5 and about 15 mole percent Al 2  O 3 , between about 35 and about 50 mole percent P 2  O 5  and between about 5 and about 15 mole percent of one of PbO, BaO, and mixtures thereof. The composition, which may also include between 0 and about 5 mole percent Fe 2  O 3  and between 0 and about 10 mole percent B 2  O 3 , has a thermal expansion coefficient in a range of between about 160 and 210×10-7/°C. and a dissolution rate in a range of between about 2×10 -  7 and 2×10 -9  g/cm 2  -min. This composition is suitable to hermetically seal to metallic electrical components which will be subjected to humid environments over an extended period of time.

RIGHTS OF THE GOVERNMENT

The United States Government has rights in this invention pursuant toContract No. DE-AC04-76DP000789 awarded by the U.S. Department of Energyto American Telephone & Telegraph Company.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to glass compositions and, moreparticularly, to a high thermal expansion, high durability glass forhermetically sealing to high expansion materials such as copper,stainless steels, alloys of aluminum, and copper/beryllium alloys.

2. Description of the Prior Art

It is preferable to utilize glass as the electrically insulating mediumfor metallic components employed in electrical devices such as vacuumtubes, explosive detonators, pyrotechnic devices, accelerator switches,connectors, and the like. Glass is desirable for use in theseapplications because it is capable of forming a complex mechanical sealand is characterized in having a low gas permeability, a high chemicaldurability, a high mechanical strength and a high electricalresistivity. However, if high thermal expansion metals, such as aluminumor stainless steel, are employed in electrical devices, the glassutilized as the electrically insulating medium must also have a thermalexpansion coefficient which closely matches the thermal expansioncoefficient of the high thermal expansion metal. This close match isrequired to avoid mechanical stresses on cooling and permit a hermeticseal to be formed between the metal and the glass. As is well known inthe art, forming a hermetic seal between the metal and the glass greatlyreduces the possibility of moisture-induced denigration or failure ofthe metal.

Compositions formed from glass and ceramics for sealing to molybdenumare disclosed in U.S. Pat. No. 3,957,496 and for sealing to stainlesssteel are disclosed in U.S. Pat. No. 4,135,936. These compositions eachrequire temperatures in excess of 900° C. to form the seal and devitrifythe glass. The high seal-forming temperatures of these compositionspreclude their use in practice with metals, such as aluminum, since theseal-forming temperatures are greater than the melting points of thesemetals.

U.S. Pat. No. 4,202,700 to Wilder, Jr. discloses a glassy compositionadaptable for hermetically sealing to aluminum-based alloys. Thecomposition may either be employed as a glass or a glass-ceramic andincludes from about 10 to about 60 mole percent Li₂ O, Na₂ O, or K₂ O,from about 5 to about 40 mole percent BaO or CaO, from 0.1 to 10 molepercent Al₂ O₃ and from 40 to 70 mole percent P₂ O₅. Although thiscomposition has a thermal expansion coefficient which closely matchesthe thermal expansion coefficient of stainless steel, its aqueousdurability (dissolution rate) is relatively poor. This shortcomingprecludes its use in electrical devices which require long operatinglifetimes in humid environments.

It is apparent from what is presently known in the art that a needexists generally for a glass composition which may be hermeticallysealed to high thermal expansion metals such as aluminum or stainlesssteel. In particular, a need exists for a glass composition forhermetically sealing to aluminum or stainless steel which has an aqueousdissolution rate at least an order of magnitude better than thedissolution rates of glasses presently known and utilized. In addition,the glass composition should have a thermal expansion coefficient whichclosely matches the thermal expansion coefficient of aluminum orstainless steel to permit the hermetic seal to be maintained as thealuminum or stainless steel expands and contracts, and a hermeticsealing temperature below the melting temperature of aluminum oraluminum alloys to permit the seal to be formed without melting eitherof the metals.

SUMMARY OF THE INVENTION

The present invention provides a glass composition designed to satisfythe aforementioned needs. The composition has a thermal expansioncoefficient in a range of between about 160 and about 210×10⁻⁷ /° C. topermit satisfactory hermetic sealing to high thermal expansion materialssuch as copper, stainless steels, aluminum alloys, and copper/berylliumalloys. In addition, the composition has a dissolution rate (in 70° C.water) in a range of between about 2×10⁻⁷ and 2×10⁻⁹ g/cm² -min topermit its use in electrical devices subjected to humid environmentsover long periods of time.

In accordance with the present invention, there is provided a glasscomposition for hermetically sealing to high thermal expansion metals,which includes between about 10 and about 25 mole percent Na₂ O, betweenabout 10 and about 25 mole percent K₂ O, between about 5 and about 15mole percent Al₂ O₃, between about 35 and about 50 mole percent P₂ O₅,and between about 5 and about 15 mole percent of one or both PbO andBaO. The composition may also include up to about 5 mole percent Fe₂ O₃and up to about 10 mole percent B₂ O₃.

These and other features and advantages of the present invention willbecome apparent to those skilled in the art upon a reading of thefollowing detailed description wherein there are described illustrativeembodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a family of high thermal expansionalkali aluminophosphate glass compositions for hermetically sealing tohigh expansion materials such as copper, stainless steels, aluminumalloys, and copper/beryllium alloys. In general, each of the glasscompositions comprises the following constituents: Na₂ O, K₂ O, Al₂ O₃,P₂ O₅, and one or both of PbO and BaO. Other constituents which may bepresent are Fe₂ O₃ and B₂ O₃. More particularly, each compositioncomprises between about 10 and about 25 mole percent Na₂ O, betweenabout 10 and about 25 mole percent K₂ O, between about 5 and 15 molepercent Al₂ O₃, between about 35 and about 50 mole percent P₂ O₅, andbetween about 5 and about 15 mole percent of one or both PbO and BaO.Each glass composition may also comprise up to 5 mole percent Fe₂ O₃ andup to 10 mole percent B₂ O₃. These glass compositions have a thermalexpansion coefficient in a range of between about 160 and 210×10⁻⁷ /° C.measured from room temperature to 325° C., a glass dissolution rate in arange of between about 2×10⁻⁷ and 2×10⁻⁹ g/cm² -min in 70° C. deionizedwater and a transition temperature in a range of between about 350° andabout 410° C.

Preferred compositions within the above component ranges can bedescribed as follows:

Glass composition I comprises 18 mole percent Na₂ O, 19 mole percent K₂O, 9 mole percent BaO, 5 mole percent Fe₂ O₃, 12 mole percent Al₂ O₃,and 37 mole percent P₂ O₅. This composition has a thermal expansioncoefficient of about 190×1-⁻⁷ /° C. measured from room temperature to325° C., a glass dissolution rate of about 4×10⁻⁹ g/cm² -min in 70° C.deionized water, a transition temperature of about 364° C. and acrystallization temperature of about 526° C. As known in the art, thedifference between the crystallization temperature and the transitiontemperature (delta T) is representative of the glass formability. Agreater (delta T) generally indicates that the glass is less prone tocrystallization during high temperature processing, including hermeticsealing. It has been found that this composition is particularlysuitable for hermetically sealing to stainless steel and Cu/Be alloys.

Glass composition II comprises 18 mole percent Na₂ O, 18 mole percent K₂O, 9 mole percent BaO, 5 mole percent Fe₂ O₃, 9 mole percent Al₂ O₃, and45 mole percent P₂ O₅. This composition has a thermal expansioncoefficient of about 203×10⁻⁷ /° C. measured from room temperature to325° C., a glass dissolution rate of about 2×10⁻⁷ g/cm² -min in 70° C.deionized water, a transition temperature of about 361° C. and acrystallization temperature of about 562° C. This composition is alsosuitable for hermetically sealing to aluminum alloys.

Glass composition III comprises 18 mole percent Na₂ O, 19 mole percentK₂ O, 9 mole percent PbO, 5 mole percent Fe₂ O₃, 6 mole percent B₂ O₃, 6mole percent Al₂ O₃ and 37 mole percent P₂ O₅. This composition has athermal expansion coefficient of about 187×10⁻⁷ /° C. measured from roomtemperature to 325° C., a glass dissolution rate of about 2×10⁻⁷ g/cm²-min in 70° C. deionized water, a transition temperature of about 372°C., and a crystallization temperature of about 587° C. Like glasscompositions I and II, glass composition III is suitable forhermetically sealing to aluminum alloys.

Glass composition IV comprises 18 mole percent Na₂ O, 19 mole percent K₂O, 9 mole percent BaO, 4 mole percent Fe₂ O₃, 10 mole percent Al₂ O₃ and40 mole percent P₂ O₅. This composition has a thermal expansioncoefficient of about 187×10⁻⁷ /° C. measured from room temperature to325° C., a glass dissolution rate of about 6×10⁻⁹ g/cm² -min in 70° C.deionized water, a transition temperature of about 382° C., and acrystallization temperature of about 560° C. Composition IV is suitablefor hermetically sealing to aluminum alloys.

Glass composition V comprises 18 mole percent Na₂ O, 19 mole percent K₂O, 9 mole percent PbO, 6 mole percent B₂ O₃, 8 mole percent Al₂ O₃, and40 mole percent P₂ O₅. This composition has a thermal expansioncoefficient of about 200×10⁻⁷ /° C. measured from room temperature to325° C., a glass dissolution rate of about 8×10⁻⁸ g/cm² -min in 70° C.deionized water, a transition temperature of about 371° C., and acrystallization temperature of about 581° C. This glass composition hasbeen found to be particularly suitable for hermetically sealing toaluminum alloys such as Al-4043.

Glass composition VI comprises 17.3 mole percent Na₂ O₃, 18.2 molepercent K₂ O, 8.6 mole percent PbO, 4 mole percent Fe₂ O₃, 5.8 molepercent B₂ O₃, 7.7 mole percent Al₂ O₃, and 38.4 mole percent P₂ O₅.This composition has a thermal expansion coefficient of about 178×10⁻⁷/° C. measured from room temperature to 325° C., a glass dissolutionrate of about 6×10⁻⁹ g/cm² -min in 70° C. deionized water, a transitiontemperature of about 382° C., and a crystallization temperature of about603° C. This composition is particularly suitable for hermeticallysealing to stainless steel alloys such as 300 series stainless steel andto Cu/Be alloys.

Glass composition VII comprises 15 mole percent Na₂ O, 18 mole percentK₂ O, 9 mole percent PbO, 6 mole percent B₂ O₃, 12 mole percent Al₂ O₃,and 40 mole percent P₂ O₅. This composition has a thermal expansioncoefficient of about 179×10⁻⁷ /° C. measured from room temperature to325° C., a glass dissolution rate of about 3×10⁻⁹ g/cm² -min in 70° C.deionized water, a transition temperature of about 391° C., and acrystallization temperature of about 631° C. Like composition VI,composition VII is particularly suitable for hermetically sealing tostainless steel alloys such as 300 stainless steel and to Cu/Be alloys.

Each of these preferred glass compositions may be made by conventionalmelt techniques. Raw materials are melted in crucibles in air attemperatures around 1200° C. for up to 5 hours. Alumina cruciblescontaminate the glass. Platinum crucibles are preferred.

The thermal expansion coefficients of the glass compositions, determinedby dilatometry, were calculated from the total glass expansion measuredfrom room temperature to just below the glass transition temperature.The dissolution rates were determined from weight changes, normalized tosample surface areas, after specified times in 100 ml of deionized waterat 70° C.

Each of the glass compositions described herein has been hermeticallysealed to either stainless steel, Cu/Be alloys or aluminum alloys.Although a certain degree of reactivity at the glass/alloy interface hasbeen noted resulting in the formation of bubbles and other reactionproducts, the development of these interfacial heterogeneities does notpreclude the formation of satisfactory glass/alloy hermetic seals. Inaddition, each of these glasses has significantly better aqueousdurability than presently known high thermal expansion phosphate-basedglasses and thus can be used in hermetic sealing applications requiringextended operating lifetimes.

It is thought that the present invention and many of its attendantadvantages will be understood from the foregoing description and it willbe apparent that various changes may be made in the form of theinvention described herein without departing from its spirit and scopeor sacrificing all of its material advantages, the forms hereinbeforedescribed being preferred or exemplary embodiments thereof.

We claim:
 1. A glass composition for hermetically sealing to highexpansion metals comprising between about 10 and about 25 mole percentNa₂ O, between about 10 and about 25 mole percent K₂ O, between about 5and about 15 mole percent Al₂ O₃, between about 35 and about 50 molepercent P₂ O₅, Fe₂ O₃ in a concentration not exceeding 5 mole percent,and between about 5 and about 15 mole percent of one of PbO, BaO and amixture thereof.
 2. A glass composition for hermetically sealing to highexpansion metals comprising between about 10 and about 25 mole percentNa₂ O, between about 10 and about 25 mole percent K₂ O, between about 5and about 15 mole percent Al₂ O₃, between about 35 and about 50 molepercent P₂ O₅, B₂ O₃ in a concentration not exceeding 10 mole percent,and between about 5 and about 15 mole percent of one of PbO, BaO and amixture thereof.
 3. The composition as recited in claim 1, wherein saidcomposition includes Fe₂ O₃ in a concentration not exceeding 5 molepercent, and further comprises 10 mole percent B₂ O₃ in a concentrationnot exceeding 10 mole percent.
 4. The glass composition as recited inclaim 1, wherein said composition comprises 18 mole percent Na₂ O, 19mole percent K₂ O, 9 mole percent BaO, 5 mole percent Fe₂ O₃, 12 molepercent Al₂ O₃ and 37 mole percent P₂ O₅.
 5. The glass composition asrecited in claim 1, wherein said composition includes 18 mole percentNa₂ O, 18 mole percent K₂ O, 9 mole percent BaO, 5 mole percent Fe₂ O₃,9 mole percent Al₂ O₃ and 45 mole percent P₂ O₅.
 6. The glasscomposition as recited in claim 3, wherein said composition includes 18mole percent Na₂ O, 19 mole percent K₂ O, 9 mole percent PbO, 5 molepercent Fe₂ O₃, 6 mole percent B₂ O₃, 6 mole percent Al₂ O₃ and 37 molepercent P₂ O₅.
 7. The glass composition as recited in claim 1, whereinsaid composition includes 18 mole percent Na₂ O, 19 mole percent K₂ O, 9mole percent BaO, 4 mole percent Fe₂ O₃, 10 mole percent Al₂ O₃ and 40mole percent P₂ O₅.
 8. The glass composition as recited in claim 2,wherein said composition includes 18 mole percent Na₂ O, 19 mole percentK₂ O, 9 mole percent PbO, 6 mole percent B₂ O₃, 8 mole percent Al₂ O₃and 40 mole percent P₂ O₅.
 9. The glass composition as recited in claim3, wherein said composition includes 17.3 mole percent Na₂ O, 18.2 molepercent K₂ O, 8.6 mole percent PbO, 4 mole percent Fe₂ O₃, 5.8 molepercent B₂ O₃, 7.7 mole percent Al₂ O₃ and 38.4 mole percent P₂ O₅. 10.The glass composition as recited in claim 2, wherein said compositionincludes 15 mole percent Na₂ O, 18 mole percent K₂ O, 9 mole percentPbO, 6 mole percent B₂ O₃, 12 mole percent Al₂ O₃ and 40 mole percent P₂O₅.